The invention relates to a laser cutting method for cutting stainless steel using a laser source of the ytterbium-doped fiber type.
At the present time, laser cutting using a laser source of the CO2 type to generate a laser beam, with a wavelength of 10.6 μm and a power ranging up to 6 kW, is widely used in industry. This method is used in particular for cutting stainless steels.
However, the cutting speeds that can be achieved and the cutting quality that results therefrom are very variable, depending on the material to be cut and, moreover, depending on the cutting method parameters adopted, such as the nature of the assistance gas, the diameter of the focused beam, the power of the incident laser, etc.
Thus, CO2 lasers cannot be used with assistance gases of low-ionization potential, for example such as argon, without the risk of generating parasitic plasmas that could impair the method.
Furthermore, CO2 lasers are limited in terms of power, thereby directly impacting the cutting speed.
In addition, the fact of having to guide the laser beam from the laser generator right to the focusing head, that is to say the cutting head, has drawbacks, especially as regards alignment of the optics in the optical path. This is because guiding optics are generally polished and/or coated copper mirrors and the positions of the latter determine the path followed by the laser beam. Therefore, the alignment of the mirrors must be perfect in order to ensure optimum entry of the laser beam into the focusing head or cutting head. Now, the position of these mirrors is generally adjusted by mechanical means, which may easily go out of alignment according to time, the wear of parts and the environmental conditions, in particular the ambient temperature, moisture content, etc.
In addition, the optical path of the beam must necessarily be kept in an inert atmosphere in order to avoid any contamination and to maintain a medium with a constant optical index, which is necessary for good propagation of the beam. These conditions make it possible for the properties relating to the beam diameter and the transverse distribution of the beam energy, and for the beam quality properties, to remain satisfactory for the method, the quality factor for beam parameter product (BPP) of the high-power CO2 laser beams used in cutting generally being between 3 mm·mrad and 6 mm·mrad. Such an atmosphere also makes it possible to preserve the guiding optics and to prevent them from deteriorating.
Now, this is not practical in an industrial situation and incurs additional costs.
In an attempt to alleviate these problems, it has been proposed to cut stainless steel with a laser device of the Nd:YAG type within which the beam is generated by a resonator containing a solid amplifying medium, that is to say a neodymium (Nd)-doped YAG rod, and sent via an optical fiber to the focusing head.
However, this solution is not entirely satisfactory from the industrial standpoint either.
This is because it has been found that cutting with a laser beam obtained with an Nd:YAG laser source with a wavelength of 1.06 μm gives poor results in terms of cutting quality and cutting speed.
This is because Nd:YAG-type lasers have quality factors unsuitable for the laser cutting process. The quality factors (BPP values) of these lasers are typically in the range from around 15 mm·mrad to 30 mm·mrad, depending on the source. Now, the higher the quality factor of a laser, i.e. the higher the product of the focused beam waist multiplied by the beam divergence, the less effective the laser beam for the laser cutting process.
In addition, the transverse energy distribution in a focused Nd:YAG laser beam is not Gaussian but has a top-hat profile, while beyond the focal point the transverse energy distribution is random.
More generally, to cut stainless steel by laser cutting with an Nd:YAG laser is far from being obvious when it is desired to achieve cutting speeds and cutting qualities that are acceptable from the industrial standpoint.
The problem that arises is therefore how to provide an improved and industrially acceptable method for cutting stainless steels with a laser beam, which can achieve, depending on the thickness in question, speeds ranging up to 15 to 20 m/min, or even higher, and good cutting quality, that is to say straight cutting faces, no burrs and reduced roughness.